STRUCTURE AND REACTIVITY OF THE ASPHALTENE FRACTION ...

t I

STRUCTURE AND REACTIVITY OF THE ASPHALTENE FRACTION OF AN ARABIAN LIGHTMEDIUM CRUDE MIXTURE Masakatsu Nomura", Koh Kidena", Satoru Mmtaa, Yan Sua, Levent Artokb, and Yasuhiro Miyatani" a

Department of Molecular Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, JAPAN

' Department of Chemistry, Cukurova University, Adana 01330, TURKEY

I

Keywords: Asphaltene, Structure, Cracking reaction

INTRODUCTION

i 1

In the petroleum industry, further utilization of distillation end points (i.e. residua) is of high interest because petroleum refineries will have to deal with much heavier crude in the future decades. Petroleum asphaltenes, which are operationally defined as pentane- or heptane-insoluble and toluene-soluble organic materials of crude oil or the bottoms from a vacuum still, are the heaviest fraction of the crude oil, and their amounts and smctures are known to be source dependent. In upgrading p r k s e s of residua, asphaltenes are responsible in sludge formation due to their flocculation, which reduces the flow and plugs down stream separators, exchangers, and towers. They show bad behavior in poisoning and reducing the activity of hydrocracking catalysts with its high heteroatom content, trace metals, and high tendency in coke formation. In order to overcome their problematic issues, the role of asphaltenic materials in the upgrading processes should also be interpreted at the level of molecule. Under these circumstances, the better comprehension of asphaltene structure is essential. Although enormous amount of effort has been paid to the structural elucidation of asphaltenes for several decpdes, their precise molecular description does not exist yet. On the basis of detailed NMR work along with complementary information from various analytical techniques employed, many researchers have concluded that asphaltenes are the mixture of polydispersed-condensed polyaromatic units, with heteroatoms contents, bearing alicyclic sites, and substituted and connected with each other via aliphatic chains. In their researches, asphaltenes were precipitated either from the crude sample or residue. The latter type asphaltene structurally may be different from the former type because at distillation temperatures, in general 300-500"C. some extent of cracking and condensation reactions may take place simultaneously. There are a number of studies which have postulated chemical models for asphaltenes, the most recent ones being based on the 'W"C NMR data and elemental composition. The models, in general, consist of one or two units of polyaromatic units in varying condensation degree combined with alicyclic sites and connected by aliphatic chains, most of the aliphatic chains being attached to the aromatic carbons[l-7]. Some researchers have used degradative methods such as pyrolysis and oxidation methods to gain more precise insight into the molecular characteristics of asphaltenes. The former method involves formation of smaller fragments and accompanies their identification, the identified components being considered as covalently bonded moieties of asphaltene molecules[8]. Strausz et al. were the frst group applied the ruthenium-ions caulyzed oxidation (RICO) reaction to asphaltenes to recognize aliphatic types[9]. They processed the invaluable information from the RICO reaction along with those from NMR and pyrolysis studies to comprehend the structure of Alberta oil sand asphaltenes and consequently proposed a very different model structure: instead of a single condensed aromatic system with a large number of rings, a set of smaller aromatic units, heteroaromatics and naphthenic units with aliphatic substituents linked by aliphatic bridges comprised the structure. Particularly, the presence of relatively polymeric naphthenic and aliphatic sites in this molecule is a striking feature. The structure of heavy fraction of crude oil and their conversion to value products have also been of our interest. In this paper, we have processed the information from the NMR work of the asphaltene sample together with data from the RICO reaction of the asphaltene to elucidate the distribution of the aliphatic carbons more precisely. The detailed analytical information over this sample is summarized within a model structure. As to the reactivity of the asphaptene, its hydrocracking reaction using metal loaded Y-type zeolite catalyst was elucidated, the results being compared with the case of the other lighter fractions. ,

EXPERIMENTAL SECTION Samples. The propane insoluble fraction of the vacuum residue of Arabian crude mixture (80% light and 20% medium) was provided from Nippon Oil Ltd. Co. The asphaltene sample used in this study was the insoluble fraction (21wt% yield, based on propane insoluble) from the pentane Soxhlet extraction of the provided sample. The elemental composition of the asphaltene sample is 83.7% C, 7.5% H , 0.84% N, 6.8% S , 0.012% Ni, 0.038% V, on dry basis and has an H/C atomic ratio of a b o h 1.ox.

Analysis of the asphaltene sample. The RICO reaction was performed by stirring the mixture of the asphaltene sample (1 9). H 0 (30 ml), CCl, (20 m!), CH,CN (20 ml), NaIO, (15 g), and RuCI,*nH,O (40 mg) at 40'C for224 h. During the reamon, N, gas was flowed and the resulting CO, was purged through CaCl, and ascarite containing tubes. The amount of CO, formed was determined from the weight increase of ascarite. The details of the workup procedure have been given elsewhere[lO]. NMR analyses were conducted by a JEOL JNM-GSX-400 spectrometer operating at 400 MHz for 'HNMR and 100 MHz for ''C NMR measurements. The NMR samples were prepared by mixing approximately 100 mg of the sample with 1 ml of C m I , ; tetramethylsilane (Th4.S) was used as an internal standard. The quantitative ',C NMR measurements were acquired by adding a relaxation agent, chromium 755

trisacetylacetonate(Cr(acac),, 0.2 M) in inverse gated decoupling system with a pulse delay of 5 s, acquisition time of 1.088 s and pulse width of 3.3 ps. The distortionless enhancement by polarization transfer ( D E W spectra were collected for flipmgles of 45'. 90". and 135". The acquisition time was the same as those for the quantitative carbon runs. A pulse delay of 2 s and a carbon-proton coupling constant of 125 Hz were used. The carbon 90" pulse was 10 ps, while proton 90" pulse was 26.3 ps. The GPC tests of the THF or CHC!, solutions of the asphaltene (0.5 mg/ml and 1.4 m g / d , respectively) were performed by a Shimadzu system with 1 ml/min flow rate of THF or CHCI, carrier solvents, respectively, at a UV wavelength of 270 tun. The columns used in these tests were Shodex KF-802 and Shodex AC-802 for THF and CHCI, carrier solvents, respectively. Standard polystyrene samples were used for the calibration of relationships between molecular weight and retention time. MALDI-TOFM (Matrim assisted lader desorption ionization-time of flightfmass spectroscepy) spectra were obtained by a Voyager RP mass spectrometer of Perspective Biosystems Co. The linear TOF mode was used with an accelerating voltage of 30 kV in positive ion. One pl of THF solution of the sample with 2.5 pg/ml concentration was applied to target and let it evaporate at atmospheric condition. Hydrocracking reaction of the asphaltene sample. A mixture of heavy oil (1.0 g) and metal-loaded zeolite (0.5 g) was placed in a 7 0 ml SUS316 autoclave, which was pressurized up to 6.9 MPa with cold hydrogen, and heated up to a desired temperature range (400OC) at a heating rate of 8Wmin. Before collection of the gaseous products, these were passed through aqueous iodine solution (1 mov1, 20 ml) to recover hydrogen sulfide produced. Gaseous hydrocarbon products were analyzed quantitatively by a gas chromatograph. The iodine solution was diluted to 200 ml by deionized water, 20 ml of which was submitted to titration with sodium thiosulfate solution (0.1 moM), using aqueous starch solution as an indicator. After the collection of gaseous products, the autoclave was opened, the inside of which was then washed with tetrahydrofuran (THF) to recover liquid and solid products. After the filtration of the resulting mixture to remove coke and catalyst, the products were separated into three fractions, gasoline + THF (the fraction distilled off by a rotary evaporator at 65°C. atm. 5 mmHg), gas oil (the fraction distilled off by a glass tube oven at 150°C. atm. 5 mmHg, corresponds to the fraction with bp < 310"C), and residue (THF soluble and undistilled fraction). Due to the severe difficulty in weighing of THF plus gasoline fraction in an accurate way the yield of gasoline fraction was calculated based on subtraction of the yield of the other fractions (gas, light oil, residue, hydrogen sulfide, and coke) from total of 100%. Amounts of coke were estimated based on the weight and elemental analysis of THF insoluble portion.

RESULTS AND DISCUSSION Structural analysis of the asphaltene RICO reaction afforded the acid products from the aliphatic portion of the sample. The amount of lower carboxylic acids (C,-C,) was 3.9 moYl00molC in asphaltene, corresponding to -5.5 aliphatic carbod 100C. Figure I shows the distribution of aliphatic monoacids up to q8 including C,-C, acids. The amount of longer n-alkanoic acids showed a smooth decrease as the carbon number increased. Therefore, the most of the alkyl groups attached to the aryl carbon or the monomethylene bridge carbon are in the range of C,-C,. We recovered the diacids ranging from C, to C,, from aqueous and dichloromethane (DCM) soluble phases of the product. Their distribution was shown in Figure 2. These acids represent alkyl bridge structures connecting two aryl units and a,a,w-triaryl substituted bridges, however, short chain acids (C,-C,) may also arise from oxidation of various hydroaromatic structures. Several amounts of ethanedioic acids were detected, this acid representing biaryl linkages in the sample. Although this acid implies the significance of biaryl linkages, its amount can not represent the amount of such type of bond due to its relative instability. Propanedioic acid could not be observed, because it can not survive the RICO reaction. Therefore, no direct evidence could be obtained from the reaction products. We also recovered the aliphatic polycarboxylic acids which were formed from thm or more aryl-substituted alkyl bridges or alicyclic parts of partially saturated condensed structures. Other polymeric aliphatic fraction could not be analyzed by GC, but was analyzed by NMR in detail. The weight of this fraction was >90% of the DCM extract. The weight and elemental composition of DCM soluble fraction indicates that the amount of carbon atom in this fraction correbponds to 24.6 C per 100 C atoms in asphaltene. GPC analysis of this fraction after methylation (methyl esterification of acids) shows a number-averaged MW of 821 Da. Figure 3 shows the I3C NMR spectrum of DCM soluble fraction before methylation. This fraction had